The present invention is directed to processes for making lyocell fibers. In particular, the method includes a step for reducing the average degree of polymerization of an alkaline pulp without substantially reducing the hemicellulose or increasing the copper number of the pulp to make the fibers.
Cellulose is a polymer of D-glucose and is a structural component of plant cell walls. Cellulose is especially abundant in tree trunks from which it is extracted, converted into pulp, and thereafter utilized to manufacture a variety of products. Rayon is the name given to a fibrous form of regenerated cellulose that is extensively used in the textile industry to manufacture articles of clothing. For over a century strong fibers of rayon have been produced by the viscose and cuprammonium processes. The latter process was first patented in 1890 and the viscose process two years later. In the viscose process cellulose is first steeped in a mercerizing strength caustic soda solution to form an alkali cellulose. This is reacted with carbon disulfide to form cellulose xanthate which is then dissolved in dilute caustic soda solution. After filtration and deaeration the xanthate solution is extruded from submerged spinnerets into a regenerating bath of sulfuric acid, sodium sulfate, zinc sulfate, and glucose to form continuous filaments. The resulting so-called viscose rayon is presently used in textiles and was formerly widely used for reinforcing rubber articles such as tires and drive belts.
Cellulose is also soluble in a solution of ammonia copper oxide. This property forms the basis for production of cuprammonium rayon. The cellulose solution is forced through submerged spinnerets into a solution of 5% caustic soda or dilute sulfuric acid to form the fibers, which are then decoppered and washed. Cuprammonium rayon is available in fibers of very low deniers and is used almost exclusively in textiles.
The foregoing processes for preparing rayon both require that the cellulose be chemically derivatized or complexed in order to render it soluble and therefore capable of being spun into fibers. In the viscose process, the cellulose is derivatized, while in the cuprammonium rayon process, the cellulose is complexed. In either process, the derivatized or complexed cellulose must be regenerated and the reagents that were used to solubilize it must be removed. The derivatization and regeneration steps in the production of rayon significantly add to the cost of this form of cellulose fiber. Consequently, in recent years attempts have been made to identify solvents that are capable of dissolving underivatized cellulose to form a dope of underivatized cellulose from which fibers can be spun.
One class of organic solvents useful for dissolving cellulose are the amine-N oxides, in particular the tertiary amine-N oxides. For example, Graenacher, in U.S. Pat. No. 2,179,181, discloses a group of amine oxide materials suitable as solvents. Johnson, in U.S. Pat. No. 3,447,939, describes the use of anhydrous N-methylmorpholine-N-oxide (NMMO) and other amine N-oxides as solvents for cellulose and many other natural and synthetic polymers. Franks et al., in U.S. Pat. Nos. 4,145,532 and 4,196,282, deal with the difficulties of dissolving cellulose in amine oxide solvents and of achieving higher concentrations of cellulose.
Lyocell is an accepted generic term for a fiber composed of cellulose precipitated from an organic solution in which no substitution of hydroxyl groups takes place and no chemical intermediates are formed. Several manufacturers presently produce lyocell fibers, principally for use in the textile industry. For example, Accords, Ltd. presently manufactures and sells a lyocell fiber called Tencel(copyright) fiber.
It is believed that currently available lyocell fibers are produced from high quality wood pulps that have been extensively processed to remove non-cellulose components, especially hemicellulose. These highly processed pulps are referred to as dissolving grade or high alpha (or high xcex1) pulps, where the term alpha (or xcex1) refers to the percentage of cellulose. Thus, a high alpha pulp contains a high percentage of cellulose, and a correspondingly low percentage of other components, especially hemicellulose. The processing required to generate a high alpha pulp significantly adds to the cost of lyocell fibers and products manufactured therefrom.
For example, when the Kraft process is used to produce a dissolving grade pulp, a mixture of sodium sulfide and sodium hydroxide is used to pulp the wood. Since conventional Kraft processes stabilize residual hemicelluloses against further alkaline attack, it is not possible to obtain acceptable quality dissolving pulps, i.e., high alpha pulps, through subsequent treatment of Kraft pulp in the bleaching stages. In order to prepare dissolving type pulps by the Kraft process, it is necessary to give the raw material an acidic pretreatment before the alkaline pulping stage. A significant amount of material primarily hemicellulose, on the order of 10% or greater of the original wood substance, is solubilized in this acid phase pretreatment and thus process yields drop. Under the prehydrolysis conditions, the cellulose is largely resistant to attack, but the residual hemicelluloses are degraded to a much shorter chain length and can therefore be removed to a large extent in the subsequent Kraft cook by a variety of hemicellulose hydrolysis reactions or by dissolution.
The prehydrolysis stage normally involves treatment of wood at elevated temperature (150-180xc2x0 C.) with dilute mineral acid (sulfuric or aqueous sulfur dioxide) or with water alone requiring times up to 2 hours at the lower temperatures. In the latter case, liberated acetic acid from certain of the naturally occurring polysaccharides (predominantly the mannans in softwoods and the xylan in hardwoods) lowers the pH below 4.
Moreover, a relatively low copper number, reflective of the relative carbonyl content of the cellulose, is a desirable property of a pulp that is to be used to make lyocell fibers because it is generally believed that a high copper number causes cellulose and solvent degradation, before, during, and/or after dissolution in an amine oxide solvent. The degraded solvent can either be disposed of or regenerated; however, due to its cost it is generally undesirable to dispose of the solvent. Regeneration of the solvent suffers from the drawback that the regeneration process involves dangerous, potentially explosive conditions.
A low transition metal content is a desirable property of a pulp that is to be used to make lyocell fibers because, for example, transition metals accelerate the undesirable degradation of cellulose and NMMO in the lyocell process.
In view of the expense of producing commercial dissolving grade pulps it would be desirable to have alternatives to conventional high alpha dissolving grade pulps as a lyocell raw material. In addition, pulp manufacturers would like to minimize the capital investment necessary to produce such types of pulps by utilizing existing capital plants.
In order to control lyocell fiber properties, lyocell manufacturers utilize dopes that comprise a blend of different pulps having different ranges of average degree of polymerization values. In view of this, there is also a need for pulp manufacturers to produce pulps having an average degree of polymerization within a relatively narrow band.
Thus, there is a need for relatively inexpensive, low alpha (e.g., high yield) pulps that can be used to make lyocell fibers, for a process of making the foregoing low alpha pulps using capital equipment that is currently available to pulp manufacturers, and for lyocell fibers from the foregoing low alpha pulp. Preferably, the desired low alpha pulps will have a desirably low copper number, a desirably low lignin content and a desirably low transition metal content.
In the prior application Ser. No. 09/256,197 now U.S. Pat. No. 6,210,801, assigned to the assignee of the subject application, various methods of reducing D.P. values and copper number of a Kraft pulp are described. Such methods include treating pulp with acid, or an acid substitute, or a combination of acids and acid substitutes. Other means of treating the pulp to reduce the average D.P. of cellulose without substantially reducing the hemicellulose content described in the prior application include treatment of the pulp with steam, a combination of ferrous sulfate and hydrogen peroxide, at least one transition metal and peracetic acid, an alkaline chlorine dioxide treatment which ends acidic or a sodium hypochlorite treatment which ends near neutral. Such processes are effective at reducing the average degree of polymerization without substantially reducing the hemicellulose content, however, such processes can be expensive from a capital improvement standpoint if the existing pulp mills in which such processes are to be used are not configured to allow for the simple deployment of such processes. In the prior application, additional steps are described in order to reduce the copper number of the pulp which have been treated to reduce its average degree of polymerization without substantially decreasing the hemicellulose content. The need for this subsequent copper number reducing step arose because the methods described in the prior application for reducing the average degree of polymerization for the cellulose resulted in an increase in the copper number for the resultant pulp.
In view of environmental concerns, there has been a great interest in using bleaching agents, which reduce the amount of chlorocompounds that must be recovered from process streams. In recent years, the use of oxygen as a delignifying agent has occurred on a commercial scale. Examples of equipment and apparatus useful for carrying out an oxygen stage delignification are described in U.S. Pat. Nos. 4,295,927; 4,295,925; 4,298,426; and 4,295,926.
While the methods described in the prior application are effective at reducing the average D.P. of cellulose without substantially decreasing the hemicellulose content, a need exists for a process that does not require a separate copper number reducing step and which is readily adaptable to pulp mills that include oxygen reactors, multiple alkaline stages and/or alkaline conditions suitable for substantial D.P. reduction of bleached or semi-bleached pulp.
As used herein, the terms xe2x80x9ccomposition(s) of the present inventionxe2x80x9d, or xe2x80x9ccomposition(s) useful for making lyocell fibersxe2x80x9d, or xe2x80x9ctreated pulpxe2x80x9d refer to pulp, containing cellulose and hemicellulose, that has been treated under alkaline conditions in order to reduce the average degree of polymerization (D.P.) of the cellulose without substantially reducing the hemicellulose content of the pulp or substantially increasing the copper number for the pulp. The compositions of the present invention preferably possess additional properties as described herein.
Compositions of the present invention are compositions useful for making lyocell fibers, or other molded bodies such as films, having a high hemicellulose content, a low copper number and a narrow molecular weight distribution, including cellulose that has a low average D.P. Preferably, the cellulose and hemicellulose are derived from wood, more preferably from softwood. Additionally, the compositions of the present invention exhibit a variety of desirable properties including a low lignin content, and a low transition metal content. Compositions of the present invention may be in a form that is adapted for storage or transportation, such as a sheet, roll or bale. Compositions of the present invention may be mixed with other components or additives to form pulp useful for making lyocell molded bodies, such as fiber or films. Further, the present invention provides processes for making compositions useful for making lyocell fibers having desirable hemicellulose content and copper number, and including cellulose that has a desirable average D.P. and molecular weight distribution.
The present invention also provides lyocell fibers containing cellulose having a low average D.P., a high proportion of hemicellulose and a low copper number, a narrow molecular weight distribution, and a low lignin content. The lyocell fibers of the present invention also preferably possess a low transition metal content.
Compositions of the present invention can be made from any suitable source of cellulose and hemicellulose but are preferably made from an alkaline chemical wood pulp such as Kraft or soda, and more preferably from a Kraft softwood pulp.
Compositions of the present invention include at least 7% by weight hemicellulose, preferably from 7% by weight to about 25% by weight hemicellulose, more preferably from 7% by weight to about 20% by weight hemicellulose, most preferably from about 10% by weight to about 17% by weight hemicellulose, and cellulose having an average D.P. of from about 200 to about 1100, preferably from about 300 to about 1100, and more preferably from about 400 to about 700. A presently preferred composition of the present invention has a hemicellulose content of from about 10% by weight to about 17% by weight, and contains cellulose having an average D.P. of from about 400 to about 700. Hemicellulose content is measured by a sugar content assay based on TAPPI Standard T249 hm-85. Further, compositions of the present invention preferably have a kappa number of less than 2, preferably less than 1. Most preferably compositions of the present invention contain no detectable lignin. Lignin content is measured using TAPPI Test T236 cm-85.
Compositions of the present invention preferably have a unimodal distribution of cellulose D.P. values wherein the individual D.P. values are approximately normally distributed around a single, modal D.P. value, i.e., the modal D.P. value being the D.P. value that occurs most frequently within the distribution. The distribution of cellulose D.P. values may, however, be multimodal i.e., a distribution of cellulose D.P. values that has several relative maxima. A multimodal, treated pulp of the present invention might be formed, for example, by mixing two or more unimodal, treated pulps of the present invention that each have a different modal D.P. value. The distribution of cellulose D.P. values is determined by means of proprietary assays performed by Thuringisches Institut fur Textil-und Kunstoff Forschunge. V., Breitscheidstr. 97, D-07407 Rudolstadt, Germany.
Compositions of the present invention which have been treated to reduce their D.P. without substantially reducing the hemicellulose content of the pulp, exhibit a desirably narrow molecular weight distribution as evidenced by a differential between R10 and R18 values (xcex94R) of less than about 2.8, preferably less than about 2.0 and most preferably less than about 1.5.
Additionally, compositions of the present invention preferably have a relatively low carbonyl content as evidenced by a copper number of less than about 2.0, more preferably less than about 1.1, most preferably less than about 0.8 as measured by TAPPI Standard T430. Further, compositions of the present invention preferably have a carbonyl content of less than about 60 xcexcmol/g and a carboxyl content of less than about 60 xcexcmol/g, more preferably, a carbonyl content less than 30 xcexcmol/g and a carboxyl content less than about 30 xcexcmol/g. The carboxyl and carbonyl group content are measured by means of proprietary assays performed by Thuringisches Institut fur Textil-und Kunstoff Forschunge. V., Breitscheidstr. 97, D-07407 Rudolstadt, Germany, referred to below as TITK.
Compositions of the present invention also preferably possess a low transition metal content. Preferably, the total transition metal content of the compositions of the present invention is less than 20 ppm, more preferably less than 5 ppm, as measured by Weyerhaeuser Test Number AM5-PULP-1/6010. The term xe2x80x9ctotal transition metal contentxe2x80x9d refers to the combined amounts, measured in units of parts per million (ppm), of nickel, chromium, manganese, iron and copper. Preferably the iron content of the compositions of the present invention is less than 4 ppm, more preferably less than 2 ppm, as measured by Weyerhaeuser Test AM5-PULP-1/6010, and the copper content of the compositions of the present invention is preferably less than 1.0 ppm, more preferably less than 0.5 ppm, as measured by Weyerhaeuser Test AM5-PULP-1/6010.
Compositions of the present invention are readily soluble in amine oxides, including tertiary amine oxides such as NMMO. Other preferred solvents that can be mixed with NMMO, or another tertiary amine solvent, include dimethylsulfoxide (D.M.S.O.), dimethylacetamide (D.M.A.C.), dimethylformamide (D.M.F.) and caprolactan derivatives. Preferably, compositions of the present invention fully dissolve in NMMO in less than about 70 minutes, preferably less than about 20 minutes, utilizing the dissolution procedure described in Example 11 below. The term xe2x80x9cfully dissolvexe2x80x9d, when used in this context, means that substantially no undissolved particles are seen when a dope, formed by dissolving compositions of the present invention in NMMO, is viewed under a light microscope at a magnification of 40xc3x97 to 70xc3x97.
A first preferred embodiment of the treated pulp of the present invention is a treated Kraft pulp including at least 7% by weight hemicellulose, a copper number less than about 2.0, cellulose having an average degree of polymerization of from about 200 to about 1100, and a xcex94R less than about 2.8.
A second preferred embodiment of the treated pulp of the present invention is a treated Kraft pulp including at least 7% by weight hemicellulose, a copper number less than two, cellulose having an average degree of polymerization of from about 200 to about 1100, the individual D.P. values of the cellulose being distributed unimodally, and a xcex94R less than about 2.8.
A third preferred embodiment of the treated pulp of the present invention is a treated Kraft pulp including at least 7% by weight hemicellulose, cellulose having an average degree of polymerization of from about 200 to about 1100, a kappa number less than two, a copper number less than 0.8, and a xcex94R less than about 2.8.
Lyocell fibers formed from compositions of the present invention include at least about 5% by weight hemicellulose, preferably from about 5% by weight to about 22% by weight hemicellulose, more preferably from about 5% by weight to about 18% by weight hemicellulose, most preferably from about 10% by weight to about 15% by weight hemicellulose, cellulose having an average D.P. of from about 200 to about 1100, more preferably from about 300 to about 1 100, most preferably from about 400 to about 700, and a lignin content providing a kappa number less than about 2.0 and more preferably less than about 1.0. Additionally, preferred lyocell fibers of the present invention have a unimodal distribution of cellulose D.P. values, although lyocell fibers of the present invention may also have a multimodal distribution of cellulose D.P. values, i.e., a distribution of cellulose D.P. values that has several relative maxima. Lyocell fibers of the present invention having a multimodal distribution of cellulose D.P. values might be formed, for example, from a mixture of two or more unimodal, treated pulps of the present invention that each have a different modal D.P. value.
Preferred lyocell fibers of the present invention have a copper number of less than about 2.0, more preferably less than about 1. 1, most preferably less than about 0.8 as measured by TAPPI Standard T430. Further, preferred lyocell fibers of the present invention have a carbonyl content of less than about 60 xcexcmol/g and a carboxyl content of less than about 60 xcexcmol/g, more preferably a carbonyl content less than about 30 xcexcmol/g and a carboxyl content of less than about 30 xcexcmol/g. The carboxyl and carbonyl group content are measured by means of proprietary assays performed by Thuringisches Institut fur Textil-und Kunstoff Forschunge. V., Breitscheidstr. 97, D-07407 Rudolstadt, Germany. Additionally, preferred lyocell fibers of the present invention have a total transition metal content of less than about 20 ppm, more preferably less than about 5 ppm, as measured by Weyerhaeuser Test Number AM5-PULP-1/6010. The term xe2x80x9ctotal transition metal contentxe2x80x9d refers to the combined amount, expressed in units of parts per million (ppm), of nickel, chromium, manganese, iron and copper. Preferably the iron content of lyocell fibers of the present invention is less than about 4 ppm, more preferably less than about 2 ppm, as measured by Weyerhaeuser Test AM5-PULP-1/6010, and the copper content of lyocell fibers of the present invention is preferably less than about 1 ppm, more preferably less than about 0.5 ppm, as measured by Weyerhaeuser Test AM5-PULP-1/6010.
Preferred embodiments of the lyocell fibers of the present invention possess desirable elongation properties. Preferably, lyocell fibers of the present invention possess a dry elongation of from about 8% to about 17%, more preferably from about 12% to about 15%. Preferably, lyocell fibers of the present invention possess a wet elongation of from about 12% to about 18%. Elongation is measured by means of proprietary assays performed by Thuringisches Institut fur Textil-und Kunstoff Forschunge. V., Breitscheidstr. 97, D-07407 Rudolstadt, Germany. Lyocell fibers produced from treated pulps of the present invention have exhibited dry tenacities on the order of about 40-42 cN/tex and wet tenacities on the order of 30-33 cN/tex as measured by the proprietary assays performed by Thuringisches Institut fur Textil-und Kunstoff Forschunge. V., Breitscheidstr.
In another aspect, the present invention provides processes for making compositions of the present invention that can, in turn, be formed into lyocell molded bodies, such as fibers or films. In this aspect, the present invention provides a process that includes contacting an alkaline pulp comprising cellulose and at least about 7% hemicellulose under alkaline conditions with an amount of an oxidant sufficient to reduce the average D.P. of the cellulose to within the range of from about 200 to about 1100, preferably to within the range of from about 300 to about 1100, more preferably to within the range of from about 400 to about 700, without substantially reducing the hemicellulose content or increasing the copper number. Pulps which are to be treated according to the present invention with an oxidant to achieve the D.P. reduction without substantially reducing the hemicellulose content or increasing the copper number as discussed above preferably have a kappa number less than 40, more preferably less than 30 and most preferably less than 25 when they are contacted for the first time with the oxidant.
This D.P. reduction treatment can occur after the pulping process and before, during or after the bleaching process, if a bleaching step is utilized. The oxidant under alkaline conditions is any oxidant containing a peroxide group such as hydrogen peroxide, oxygen, chlorine dioxide and ozone. Preferably the oxidant is a combination of oxygen and hydrogen peroxide, or hydrogen peroxide alone.
Preferably the carbohydrate yield of the D.P. reducing step of the present invention is greater than about 95%, more preferably greater than about 98%. The process yield is the dry weight of the treated pulp produced by the process divided by the dry weight of the starting material pulp, the resulting fraction being multiplied by one hundred and expressed as a percentage.
In another aspect of the present invention a process for making lyocell fibers includes the steps of (a) after the pulping process, contacting an alkaline pulp including cellulose and at least about 7% hemicellulose with an amount of an oxidant sufficient to reduce the average degree of polymerization of the cellulose to the range of from about 200 to about 1100, preferably to the range of from about 300 to about 1100, without substantially reducing the hemicellulose content or increasing the copper number of the pulp; and (b) forming fibers from the pulp treated in accordance with step (a). In accordance with this aspect of the present invention, the lyocell fibers are preferably formed by a process selected from the group consisting of melt blowing, centrifugal spinning, spun bonding and a dry jet/wet process.